Artificial Photosynthesis Using Titanium, Zirconium, and Hafnium Tetrahalide Complexes with Visible-Light-Active Chromophores, Including 2-Phenyl Indole and 2-Phenyl Benzoxazole

This application claims priority from U.S. Prov. Patent App. No. 63/566,874 entitled “ARTIFICIAL PHOTOSYNTHESIS WITH TITANIUM, ZIRCONIUM AND HAFNIUM OXYHALIDES/2-PHENYL INDOLE PHOTOCATALYTIC COMPLEXES”, filed Mar. 18, 2024, the entire disclosure of which is hereby incorporated by reference in its entirety.

The present invention relates to the field of artificial photosynthesis (AP), specifically involving visible-light photocatalytic complexes of titanium, zirconium, and hafnium tetrahalides in combination with 2-phenyl indole and related chromophores. These complexes can directly capture atmospheric carbon dioxide (CO) and convert it into oxygenated hydrocarbons with linear carbon chains ranging from Cto C. Additionally, the photocatalytic system absorbs atmospheric humidity, facilitating the splitting of water (HO) into hydrogen, which is utilized in the reduction of CO, as well as the formation of hydrogen peroxide (HO).

“Living” chemical systems autonomously self-organize, perform complex tasks, and harvest energy from inexhaustible sources like sunlight, air, and water. Such systems, particularly photocatalysts operating away from equilibrium under constant energy and material input, embody artificial photosynthesis's fundamental concept and goal (AP). Inspired by natural photosynthesis, AP seeks to create efficient systems that produce valuable products while offering immense environmental, energy, and economic benefits.

Photocatalytic water splitting, using catalysts such as titanium dioxide (TiO), has been extensively studied due to TiO's stability, low cost, and eco-friendliness. However, its efficiency requires significant improvements to achieve commercial viability. Current hydrogen production predominantly relies on indirect methods like solar-driven electrolysis. Meanwhile, direct air capture (DAC) of COremains energy-intensive, often requiring chemical reactants and high energy input. No reported system-apart from natural photosynthesis-simultaneously captures and reduces COdirectly from the atmosphere under ambient conditions. Existing COreduction methods typically require concentrated COsources and complex devices, limiting scalability and sustainability.

A more sustainable approach would integrate water splitting and COreduction into a single, continuous process to produce valuable organic compounds beyond simple molecules like CO, CH, or CHOH. Current catalytic methods are hindered by complex setups, high energy requirements, and poor reproducibility. Despite decades of research on TiO, molecular-level titanium photocatalysts remain underexplored. While surface-bound functionalities like Ti═O, Ti—O—O, and Ti(OH)enhance bulk TiOphotocatalysis, molecular titanium complexes, such as titanium oxide dichloride (ClTi═O), offer a promising avenue for improving photocatalytic efficiency through ligand-to-metal charge transfer (LMCT) photochemistry.

Titanium-, zirconium-, and hafnium-based coordination compounds are abundant, stable, and well-suited for photocatalytic applications. When paired with visible-light-sensitive chromophores, such as 2-phenyl indole and 2-phenyl benzoxazole, these metal halide complexes can form self-organizing systems that harvest light energy and facilitate complex chemical reactions. However, their full potential for autonomous COcapture and conversion under ambient conditions has not yet been realized.

This invention introduces a novel artificial photosynthesis system utilizing titanium, zirconium, and hafnium tetrahalide complexes with visible-light-active chromophores. Operating under ambient conditions, the system directly captures atmospheric COand water vapor, converting them into long-chain oxygenated hydrocarbons (Cto C) and producing hydrogen peroxide (HO) as a valuable byproduct. This self-sustaining process leverages a novel self-catalyzed oligomerization mechanism to create complex organic products, representing a significant advancement in green chemistry and renewable energy technologies.

The present invention provides a groundbreaking method for artificial photosynthesis, harnessing visible light to directly capture atmospheric carbon dioxide (CO) and water (humidity) under ambient conditions. This process utilizes self-organizing titanium, zirconium, and hafnium tetrahalide complexes in combination with visible-light-active chromophores, such as 2-phenyl indole and 2-phenyl benzoxazole, which together form a robust photocatalytic system. The system performs continuous, autonomous chemical operations, converting COinto long-chain oxygenated hydrocarbons with carbon chains ranging from Cto C. These hydrocarbons have significant potential in renewable energy applications, as well as in the production of specialty chemicals and biofuels.

In addition to COcapture and conversion, the photocatalytic system absorbs atmospheric humidity, splitting water (HO) into hydrogen, which is subsequently used to reduce CO, as well as generating hydrogen peroxide (HO) as a byproduct. This dual function—simultaneously reducing COand splitting water—enhances the efficiency of the system, making it highly effective for energy and environmental applications.

The process operates through a novel, self-catalyzed oligomerization mechanism that allows initially produced Cmaterials to oligomerize into larger organic molecules. This system's self-organizing behavior distinguishes it from traditional catalytic systems by enabling continuous product formation without the need for external catalysts or interventions. The products formed in the system include linear hydrocarbons, ranging from simple Cunits to complex long-chain organic molecules, expanding the range of potential applications for artificial photosynthesis technologies.

The chemical transformations within the system have been characterized using high-resolution MALDI-TOF (matrix-assisted laser desorption ionization-time of flight) mass spectrometry, which enables accurate identification of molecular structures and product distribution. Complementary infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy are used to confirm the identities and purity of the generated compounds, ensuring the reliability of the system's output.

To provide a clearer understanding of the invention, reference is made to the accompanying drawings and detailed description. These illustrate the structural and operational aspects of the self-organizing artificial photosynthesis system, highlighting key components such as the 2-phenyl indole (PI) or 2-phenyl benzoxazole (BX)/titanium tetrachloride complexes, the sequential chemical transformations, and the pathways leading to the formation of long-chain oxygenated hydrocarbons. Together, these sections offer a comprehensive overview of the invention's mechanisms and outcomes.”

While this disclosure is susceptible of embodiment in many different forms, there is shown in the drawings and described herein in detail a specific embodiment(s) with the understanding that the present disclosure is to be considered as an exemplification and is not intended to be limited to the embodiment(s) illustrated.

It will be understood that like or analogous elements and/or components, referred to herein, may be identified throughout the drawings by like reference characters. In addition, it will be understood that the drawings are merely schematic representations of the invention, and some of the components may have been distorted from actual scale for purposes of pictorial clarity.

This invention relates to artificial photosynthesis catalyzed by titanium complexes incorporating photoactive ligands such as 2-phenyl indole (PI) and 2-phenyl benzoxazole. The system operates under visible light and ambient conditions, performing direct atmospheric capture (DAC) of CO, splitting water, and producing oxygenated organic compounds, including long-chain hydrocarbons. The synergy between titanium's redox-active coordination sphere and the photoionizable ligands enables a self-sustaining catalytic cycle. Key processes include hydrolysis, COcapture, light-induced redox transformations, and product synthesis.

The catalytic system initiates when Ti—Cl bonds in the parent complex (1) are hydrolyzed by air humidity, forming Ti—OH (2) or transient Ti═O bonds (,).

These functionalities are key to enabling the visible light photocatalytic cycle. A crucial intermediate, complex 4, forms through this hydrolysis and may also be prepared directly by reacting two equivalents of PI with TiOCl.

Upon visible light irradiation, Ti—OH expels OH radicals, reducing Ti(IV) to Ti(III) and forming complex 44.

Chlorine radicals, derived from HCl interaction, contribute to para-chlorination of the PI ligand. Further hydrolysis leads to complex 45, which transforms under light to the transient Ti(II) species 46, enabling DAC of COand interactions with small molecules such as CHO, CO, and O.

The system autonomously captures and reduces atmospheric COunder ambient conditions. Complex 6 reacts with CO, forming carbonate-containing intermediates such as complex 30 and 31.

This process is one of the first examples of a mononuclear titanium complex interacting with atmospheric COto form cyclic carbonate groups. These intermediates are precursors to organic product synthesis and highlight the system's potential for sustainable COutilization.

Captured COis reduced to CO, HCHO, and CHOH using system-generated Hor via hydrogen transfer from water.

Formaldehyde and methanol couple photocatalytically to form ethylene glycol (HOCHCHOH), which integrates into titanium complexes.

The catalytic system's ability to utilize DAC-derived COfor synthesizing oxygenated organics showcases its potential for large-scale sustainable chemical production.

The system extends DAC-reduced formaldehyde into long-chain oxygenated hydrocarbons.

Titanium α-ketocarboxylate complex 21 catalyzes a dehydration synthesis of formaldehyde, forming ketene-containing intermediates (complex 23). Subsequent chain growth produces C2-C6 products with alpha-omega functionalities, mimicking natural photosynthetic pathways.

Ligand exchange involving PI and donor molecules (e.g., THF, HCHO) results in mixed-donor complexes.

These exchanges influence the overall catalytic efficiency by providing open coordination sites on titanium for system-generated small molecules, ensuring active participation in the reaction cycle.

Photoionized PI radicals undergo oligomerization. This process consumes PI ligands from titanium complexes, forming oligomers detected via MALDI-TOF. The implications of these oligomers on system performance and their potential electronic and fluorescent properties remain subjects for further study.

Hydrolysis products from 2:1, 1:1, and 1:2 PI/TiCl4 complexes exposed to visible light: To optimize the molar ratio of PI/TiCl4 complexes for improved product yield and distribution, we subjected complexes prepared at PI:TiCl4 molar ratios of 2:1, 1:1, and 1:2 to hot water treatment. These were exposed to visible light under the same conditions as the 2:1 complex (see). soluble and insoluble fractions were observed in all cases. MALDI-TOF analysis was performed on each fraction, with the results of the water-soluble fractions summarized in Table 1.

Table 1 presents the MALDI-TOF data in the first column, the proposed chemical product structures in the second, and the relative product distributions (on a 0-8 scale) for each complex in the subsequent three columns. The calculated molecular weights (MW) and m/z values align well with the experimentally obtained MS data for all proposed products. Notably, most products are consistent across the three complexes, highlighting the system's reproducibility. The most compelling results were observed for the 1:1 complex, which exhibited a reduced product diversity, higher overall relative yield, and a dominant product at m/z 177.0431, accounting for over 40% of the total yield. This suggests that the molar ratio in the complex significantly influences the chemical product distribution—a crucial feature of this catalytic system that warrants further investigation.

A unique peak at m/z 507.0319 was observed exclusively in the 1:1 complex. This peak is tentatively assigned to the coupling product of a C9 oxygenated species with a

chlorinated PI ligand (Arzoumanidis et al. 2024). Additionally, a geometric projection based on total product yields across the three complexes suggests an optimal PI:TiCl4 molar ratio of approximately 1.25, with an estimated relative product yield of 25 (see Table 1).

The hot water-insoluble fraction primarily consisted of PI oligomers, forming a continuous series with decreasing concentrations, containing up to 10 monomers (see). A minor sequence of m/z peaks was also observed, increased by 34 m/z units relative to the primary sequence (e.g., 418-384, 609-575, 800-766, 991-957). In each pair, the higher value corresponds to a PI oligomer incorporating a chloride substitution.

The described photocatalytic system integrates DAC of CO, water splitting, and production of valuable oxygenated organics through a series of light-driven transformations. Titanium's redox flexibility, combined with the photoactivity of PI ligands, creates a sustainable platform for artificial photosynthesis with significant environmental and economic implications.

Introduction This invention relates to the field of artificial photosynthesis and chemical catalysis, specifically the utilization of earth-abundant titanium coordination complexes to produce long-chain oxygenated organic compounds from atmospheric CO2 and water under ambient conditions. The invention leverages a self-organized, autocatalytic system energized by visible light, demonstrating unprecedented efficiency and scalability in carbon fixation and conversion.

Overview of the System The invention employs 2-phenyl indole (PI) complexes of titanium tetrachloride, represented as (PI)2TiCl4, as primary catalytic units. These complexes, when exposed to visible light, undergo hydrolysis by atmospheric humidity, forming organotitanium intermediates. These intermediates capture and reduce CO2 directly from ambient air, producing C2 to C17 oxygenated hydrocarbons.

Example 1: Preparation of (PI)2TiCl4 Complex 2-Phenyl indole (2 mmol) was dissolved in dichloromethane (10 mL), and TiCl4 (1 mmol) was added dropwise under nitrogen atmosphere. The mixture was stirred at room temperature for 2 hours, resulting in the formation of (PI)2TiCl4 as a yellow solid. The product was isolated by filtration and characterized using NMR and IR spectroscopy.

Example 2: Photocatalytic Reaction under Ambient Conditions The (PI)2TiCl4 complex (50 mg) was dispersed in water (10 mL) and exposed to visible light (400-700 nm) in an open-air setup. MALDI-TOF analysis of the reaction mixture after 24 hours revealed the formation of C6-C9 α-carboxylic acid-ω-aldehyde compounds.

Example 3: Chain Growth to C17 Hydrocarbons To a solution of the initial reaction products, additional formaldehyde (1 mmol) was introduced, and the mixture was irradiated for 48 hours. Mass spectrometry confirmed the formation of dimerized and oligomerized products up to C17.